1. Field of the Invention
This invention relates to a pile driver, or more specifically to a pile driver comprising as a base machine construction equipment having a work arm for operation against the ground and a high-frequency vibration type pile driving device attached to the end of the work arm.
2. Description of the Prior Art
As a conventional pile driver of this type, one that comprises, as a base machine, construction equipment such as an excavator having a work arm for drilling, crushing and other work against the ground, and a rotating eccentric weight type pile driving device attached at the end of the arm, is known.
This type of pile driver drives a pile into the ground under a force which is a superposition of the centrifugal force due to rotation of the eccentric weights, a portion of the gravity force acting on the base machine, and the hydraulic force from the hydraulic cylinder mounted on the base machine.
Here, description of the a pile driver of the rotating eccentric weight type will be given with reference to FIG. 1. In a casing 101, under which is provided a chucking means 107 to grip a pile K to be driven, are arranged a pair of eccentric weights 102 and 103 of the same mass m. These eccentric weights 102 and 103 are fixed on respective ones of a pair of parallel rotatable axles at a distance r from the center of gravity thereof, and are rotated by a motor 106 in opposite directions at the same rate of revolution through engagement of synchronizing gears 104 and 105 having the same number of teeth.
In this pile driver of the eccentric weight type, when the eccentric weights are rotated at an angular velocity .omega., the horizontal components of the centrifugal forces cancel out while the vertical components are added up to constitute the driving force on the pile K varying sinusoidally with the maximum value of F=2mr .omega..sup.2.
The operation principle of the conventional pile driver being as described above, in order to obtain a greater driving force, it is required, as seen from the above formula, to increase either the mass m of the eccentric weights, or the angular velocity .omega. or the eccentric throw r, which, however, means necessarily greater loads on the axles supporting the eccentric weights, on the bearings supporting the axles rotatably, and on the frame holding the bearings as well as on the power transmission mechanism. The increase in loads, in turn, means rapid frictional wear or, if the wear is to be prevented, high-strength design using members of greater dimensions.
To avoid the inconvenience described above, it is necessary to restrict the rate of revolution of the eccentric weights within a limit, which, however results in vibration of rather low frequency transmitted from the pile K through the ground. The rumbling of the ground in the neighboring areas caused by the low-frequency vibration transmitted from the driven pile, the deterioration of the base of buildings due to it, and, in the worst cases, inclination of buildings, all make the vibration a public hazard.